From the story, the author seems to focus on the bet between the banker and the young man; the significance of the bet to each of the characters is explored throughout the story.
The banker is characterized as a man who enjoys the thrill of the gamble; he is excited by the relentless pace of speculative trading and revels in the rewards it accords him. However, fifteen years of indiscriminate, wild speculation has left him almost penniless. From a fortune of millions, he now owes his last two million to the young man. The banker resolves to hold on to this money by murdering the young man. Although he is ashamed by his cowardice and lack of principles, he feels he has no choice.
The young man, however, has been spending his isolation by alternately playing the piano and reading an immense quantity of books. He studies six languages and becomes fluent enough to write in all six of them. He is ecstatic at the amount of knowledge he has been able to absorb during his voluntary confinement. However, he soon realizes that all the knowledge he has amassed is futile. He tells us why:
"And I despise your books, I despise wisdom and the blessings of this world. It is all worthless, fleeting, illusory, and deceptive, like a mirage. You may be proud, wise, and fine, but death will wipe you off the face of the earth as though you were no more than mice burrowing under the floor, and your posterity, your history, your immortal geniuses will burn or freeze together with the earthly globe..."
So, the focus of the story is the bet and what it brings about: both the young man and the banker come to the realization of a great truth. Whether one amasses wealth or knowledge, the end result is the same. One cannot retain either one when death calls. It is significant that the young man spends nearly a year reading the Gospels. The banker finds it odd that, after learning six languages, the young man would focus his attention on such simple study. However, the young man comes to recognize that he is, at heart, as much of a gambler as the banker is. Both forget to ponder the real meaning of life nor to even enjoy life itself; indeed, both focused time and energy on procuring things that could never be eternal.
"You have lost your reason and taken the wrong path. You have taken lies for truth, and hideousness for beauty...so I marvel at you who exchange heaven for earth. I don't want to understand you..."
Hope this helps!